Japanese Laid-Open Patent Publication No. 2004-256262 describes a known seal chain in which lubricating oil is sealed between the inner circumferential surface of a bushing and the outer circumferential surface of a connecting pin. The seal in such a chain comprises a ring that contacts at least an end surface of the bushing, and an elastic seal in pressing contact with the ring and an outer link plate facing the ring.
In the above-described conventional seal chain, in order to seal the lubricating oil between a pin and a bushing effectively, the ring member and elastic seal are pressed between the bushing and the outer link plate. The pressing force creates resistance to relative bending between the outer link plates and the inner link plates, and the result is an increased overall resistance of the chain to bending.
The bending resistance in the conventional chain can be reduced by surface finishing to decrease the roughness of the inner surfaces of an outer link plates and the end surfaces of the bushings. Surface finishing is also desirable in order to reduce wear of the ring and the elastic seal. However, need for surface finishing increases the cost of production of the chain.
Since the ring and the elastic seal are additional parts, the overall number of parts in the chain is increased, and assembly of the chain is made more difficult. The need for these additional parts also increases the cost of production.
Accordingly, an object of the invention is to solve the above-mentioned problems. The invention addresses these problems by providing an oil-free chain which achieves several objectives. First, it prevents leakage of lubricating oil and the invasion of external dust during the operation of the chain. Second, it suppresses wear between the outer circumferential surfaces of the connecting pins and the inner circumferential surfaces of the bushings so that wear elongation of the chain is prevented for a long period of time. Third, the number of chain parts is small in comparison with the number of parts in the above-described conventional seal chain, and the cost of production and difficulty of assembly and disassembly of the chain are reduced. The invention accomplishes these objectives while maintaining the strength of the link plates of the chain.